One method of obtaining a polycrystalline, or microcrystalline, silicon film is to irradiate a completed amorphous silicon film with laser radiation, for crystallizing the amorphous silicon. This method is generally well known. Laser-crystallized thin-film transistors fabricated by making use of this technique are superior to amorphous silicon thin-film transistors in electrical characteristics including field effect, mobility and, therefore, these laser-crystallized thin-film transistors are used in peripheral circuit-activating circuits for active liquid-crystal displays, image sensors, and so forth.
The typical method of fabricating a laser-crystallized thin-film transistor is initiated by preparing an amorphous silicon film as a starting film. This starting film is irradiated with laser radiation to crystallize it. Subsequently, the film undergoes a series of manufacturing steps to process the device structure. The most striking feature of the conventional manufacturing process is to carry out the crystallization step as the initial or an intermediate step of the series of manufacturing steps described above.
Where thin-film transistors are fabricated by this manufacturing method, the following problems take place:
(1) Since the laser crystallization operation is performed as one step of the manufacturing process, the electrical characteristics of the thin-film transistor (TFT) cannot be evaluated until the device is completed. Also, it is difficult to control the characteristics.
(2) Since the laser crystallization operation is effected at the beginning of, or during, the fabrication of the TFT, it is impossible to modify various electrical characteristics after the device structure is completed. Hence, the production yield of the whole circuit system is low.